Sample Surveys Chapter 11

Sampling Goal: Learn about the parameters of a population that largest enough that it is impossible or impractical to measure. To accomplish this we take a representative sample and use it to draw conclusions about the population.

Bias A sample that does not represent the population in some important way is biased. Any sample that systematically under or other represents a section of the population is biased. Random selection of the individuals in the sample is the best remedy for bias.

Sample Facts The size of a population is not relevant to the reliability of the sample. The sample size is the most important factor in the reliability.

Terms Census: A sample that includes the entire population. Parameter: A trait of a population. Its mean and standard deviation of the population are parameters. Statistic: Calculated values that summarize a sample. Statistics are usually used to estimate population parameters.

Parameters vs Statistics If statistics from a sample accurately reflect the parameters of the population it is said to be representative.

Simple Random Sample A SRS is one in which every possible sample of a given size has an equal chance of being selected. A sampling frame is a list of all of the individuals from which the sample is drawn.

Terms II Sampling Variability: The different results we obtain when more than one sample is drawn. It is the difference between samples.

Sampling Designs Stratified random sample: the population is divided into homogeneous groups called strata and then a SRS is taken from each strata. Forces representation different groups. Use when there are different groups within a population that might have react differently.

Sampling Designs II Cluster Sampling: the population is made up of heterogeneous groups. Randomly select group(s) and take all. Multistage Sampling: Sampling methods that combine more than one sampling method. Systematic Sampling: using a rule to select a sample. Use when the order of the list will have no effect on the responces.

Types of Bias Voluntary Response: Self selected sample. Volunteers Convenience Sampling: sample the individuals that are easy or nearby. Undercoverage: lack of representation of a section of the population. Nonresponse bias: Selected members of the sample do not reply. Response bias: Anything that influences the responses. For example: leading questions